[docs][scudo] Add information about M_PURGE_ALL.

[cferris]

Summary

Add information about M_PURGE_ALL

[cferris]

Fix typo.

[Chia-hungDuan]

Maybe we can say more about the difference between M_PURGE and M_PURGE_ALL? From the description, I'm not sure that I understand which one I should use.

[cferris]

Added more description about M_PURGE and M_PURGE_ALL.

[cferris]

See if this description is easier to understand and accurate.

[Chia-hungDuan]

LGTM, thanks!

llvm/docs/ScudoHardenedAllocator.rst

 +---------------------------+-------------------------------------------------------+
 | Option                    | Description                                           |
 +---------------------------+-------------------------------------------------------+
 | M_DECAY_TIME              | Sets the release interval option to the specified     |
 |                           | value (Android only allows 0 or 1 to respectively set |
 |                           | the interval to the minimum and maximum value as      |
 |                           | specified at compile time).                           |
 +---------------------------+-------------------------------------------------------+
-| M_PURGE                   | Forces immediate memory reclaiming (value is unused). |
+| M_PURGE                   | Forces immediate memory reclaiming but does not       |
+|                           | reclaim everything. For smaller size classes, there   |
+|                           | is still some memory that is not reclaimed due to the |
+|                           | extra time it takes and the small amount of memory    |
+|                           | that can be reclaimed.                                |
+|                           | The value is ignored.                                 |
++---------------------------+-------------------------------------------------------+
+| M_PURGE_ALL               | Same as M_PURGE but will force release all possible   |
+|                           | memory regardless of how long it takes.               |
+|                           | The value is ignored.                                 |
 +---------------------------+-------------------------------------------------------+
 | M_MEMTAG_TUNING           | Tunes the allocator's choice of memory tags to make   |
 |                           | it more likely that a certain class of memory errors  |
 |                           | will be detected. The value argument should be one of |
 |                           | the enumerators of ``scudo_memtag_tuning``.           |
 +---------------------------+-------------------------------------------------------+
 | M_THREAD_DISABLE_MEM_INIT | Tunes the per-thread memory initialization, 0 being   |
 |                           | the normal behavior, 1 disabling the automatic heap   |